Heat exchangers



Dec. 27, 1966 H. J. VIVOOD 3,294,161

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HTTORNEYJ Dec. 27, 1966 WOOD 3,294,161

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o 2 4 6 I 8 IO Homer J M/Ooa/ E-NTU FOR c Min/Max ATTORNEYS UnitedStates Patent 3,294,161 HEAT EXCHANGERS Homer J. Wood, Sherman Oaks,Calif., assignor to Continental Aviation and Engineering Corporation,Detroit, Mich., a corporation of Virginia Filed July 3, 1961, Ser. No.121,748 6 Claims. (Cl. 165-140) My invention relates to heat exchangersand more particularly to a stationary type heat exchanger in which aheat transfer fluid serves to transport heat from a hot fluid stream toan adjacent cold fluid stream preferably flowing counter to the hotfluid stream.

The present heat exchanger was developed from a concept intended forapplication as a gas turbine regenerator, and the present descriptionwill so refer to such an application, although it will be seen that manyother uses for it may be found.

Gas turbine regenerator construction must take into consideration theenvelope and flow arrangement of the power plant and in addition mustallow for loss of performance of the complete package due to any leakagebetween the highapressure side and the low-pressure side of the heatexchanger.

Engine profile is also important in many applications, notablyautomotive, where the permissible envelope is dictated by considerationsbeyond the control of the power plant designer.

For a given heat transfer problem, with fluid flow rates andeffectiveness fixed, the smallest possible heat exchanger will be of thecounterfiow configuration. Therefore, eflorts to reduce weight and bulklogically start from the counterfiow rather than the crossflowarrangement. However, ordinary direct counterfiow heat exchangers of theextended surface or compact type required for a regenerator, presentextremely complex ducting problems.

Other eflForts to reduce the-size and Weight of regenerators haveresulted in the evolution of the rotary counterflow regenerator. Thistype of regenerator is inherently smaller than a crossflow staticregenerator of the same effectiveness due to thermodynamicconsiderations. Furthermore, in most cases, the rotary regeneratorresults in simpler ducting for the hot and cold fluids; hence a moredesirable profile or frontal area.

The main disadvantages of the rotary regenerator are seal leakage andcarry-over, both of which result in leakage from the high to lowpressure sides of the unit with resulting loss of engine performance.

An object of the present invention is to simplify and improve heatexchangers by providing a construction which combines the best featuresof both the rotary counterfiow and static crossflow concepts.

Another object of the invention is to improve heat exchangers byeliminating the seal leakage and carry-over problems inherent in rotaryregenerators as well as the bulky ducting heretofore required in thestatic type regenerators.

A further object of the invention is to reduce weight and bulk of staticheat exchangers by constructing same using principles of rotarycounterfiow regeneration.

Yet another object of the invention is to construct a new type of heatexchanger by providing a composite of several liquid metal to gas heatexchangers with the liquid metal flowing in cross-flow to the gas and inwhich the hot gas flows in a direction counter to that of the cold gas,the hot and cold gases flowing through adjacent areas.

For a more complete understanding of the invention, reference may be hadto the accompanying drawings illustrating several embodiments of theinvention in which like reference characters refer to like partsthroughout the several views and in which FIG. 1 is a partiallysectional elevational view of a preferred gas turbine engineincorporating a regenerator system embodying the present invention.

FIG. 2 is a perspective view, partially broken away, of a heat exchangeradapted for connection with a gas turbine and embodying the presentinvention.

FIG. 3 is a vertical transverse cross-sectional view of the structure ofFIG. 2, as taken substantially on the line 33 of FIG. 2.

FIG. 4 is a cross-sectional view taken substantially on the line 4-4 ofFIG. 3.

FIG. 5 is a top view of the heat exchanger of FIGS. 2 and 3.

FIG. 6 is a fragmentary perspective view of a core section used in theheat exchanger.

FIG. 7 is a diagrammatic view of another embodiment of the invention.

FIG. 8 is a diagrammatic view of a further embodiment of the invention.

FIG. 9 is a fragmentary diagrammatic view of yet another embodiment ofthe invention.

FIG. 10 is a perspective diagrammatic view of the heat exchangerillustrated in FIG. 7.

FIG. 11 is a diagram illustrating the temperature excursion of heattransporting fluid passing through source and sink and successive planesof tubes of the present regenerator from heat source to cold end.

FIG. 12 is a cross-sectional view taken substantially on the line 12-12of FIG. 7.

FIG. 13 is a diagram illustrating the results of theoretical performanceanalysis of the present heat exchanger.

FIG. 14 is a data chart used in the analysis of the heat exchanger.

In FIG. 1, a preferred gas turbine engine 10, to which the present heatexchanger may be adapted, comprises an axial compressor structure and aradial compressor structure 12 which are operable to compress airentering the air inlet 13.

The compressed air is first conducted to a regenerator 14 where itbecomes heated, then'returned to a chamber 15 as indicated by flowarrows, and then directed into a combustor 16, and is mixed with fuel.The mixture is burned, and the hot gases are expelled to operate aturbine 17 and then exhausted through the regenerator 14.

The regenerator operates to transfer heat from the exhaust to thecompressed air. Actual construction of the gas turbine engine isimmaterial to the present invention which is directed to the regeneratoritself. It will be understood that although the regenerator to bedescribed was developed with its application to gas turbines in mind,the principles and construction to be described may have otherapplications.

The preferred regnerator 14 as illustrated in FIGS. 2-6 comprises aperipheral frame structure 20 having forward and rear flanges 21 and 22adapted for assembly in a gas turbine engine, and a reinforcing channelframe 2 3 with a peripheral casing 23A.

Within the frame structure 20, and inwardly spaced from three sidesthereof, is a core assembly 24. Spaces 25 between opposite sides of thecore assembly 24 and the sides of the frame structure and extending overportions of the core assembly as at 25A are adapted for connection withducts carrying the air from the compressor. Space 25B over the corestructure 24 may be adapted for connection with a turbine exhaustbypass.

Shroud structures 26 carried on the rear side of the frame structure 20direct the compressor air around and forward through spaced side potions24A of the core structure 24, these portions 24A being adapted forconnection to ducts carrying regenerated air back to the engine chamber15. The exhaust from the gas turbine normally flows through a centerportion 24B of the core structure 24 and in a direction counter to thedirection of air flow through the core portions 24A.

The core structure 24, as illustrated in FIG. 6, comprises a pluralityof parallel levels of flattened conduit structures 30 each having alower plate 30A and an upper plate 30B flanged as at 30C to divide thestructure into a plurality of flat broad passages 30D.

The levels of conduit structures 30 are separated by a triangularlycorrugated sheet 31 providin in effect, finned passages 31A extendingnormal to the passages 30D. It will be seen that the result of using thepresent triangularly finned structure is to provide a centralmultiapassage hot gas duct intermediate and adjacent multi-passage coldgas ducts, the ducts having common separating side walls.

A return header 35 is provided adjacent the header 33 and is dividedinto a plurality of vertical passages 35A. A multisection fluid pump 36is provided to pump the heat transfer fluid from the upper end ofindividual header passages 35A into the upper ends of the header passages 33A, from which the fluid flows into the uppermost levels of theconduit structures 30 for circulation back and forth and from top tobottom.

The heat exchange fluid used in the above-described regenerator ispreferably a liquid metal such as bismuthlead tin, sodium potassium ormercury. Sodium potassium is good because of its 'low melting point (12F.), low weight and eflicient heat transport properties. Alkali metalsare generally avoided in gas turbine engine applications because oftheir violent oxidation reaction in the event of a leak.

The operation of the heat exchanger 14 will be apparent. Heat from theturbine exhaust flowing through the passages 31A in the center coresection 24B is absorbed by the cross-flowing liquid metal in thepassages 30D and transported to the side core sections 24A where theheat is given up to the air flowing counter to the exhaust flow.

The liquid metal flow is confined by the conduits 30D being individually'headered so that stratification of temperature is maintained betweenthe forward and rear faces of the core structure 34 to preserve thecounter-flow effect. The liquid metal pumping rates are not critical aslong as they are above minimum values and regulation of metal pumping isnot required. In the present regenerator 14, negligible horsepower isrequired to drive the pump 36, compared with turbine power output.

Thermal behavior of the regenerator 14 is quite similar to that of arotary regenerator, Where the liquid metal flow is analogous to a movingheat transfer matrix. However, there are no carry-over and leakagelosses as in rotary regenerators, and the only inhibition on metal flowrates is pumping power. Moreover, comparisons made show that for equaleffectiveness, pressure loss and hydraulic radii (on gas and airpassages), the above described regenerator has about 30 percent lessbulk than a rotary regenerator and equal Weight. Also, it Will be seenthat the present device has simple basic construction and is easy toclean because of the straight-through pasages.

It is also of great importance to note that the present use oftransverse metal flow avoids the problem, encountered in those heatexchangers in which gas and metal move in counterflow, of therequirement that metal flow rates be controlled and maintainedproportional to gas flow rates. In the present concept, such control isnot necessary.

FIG. 10 is a perspective view of another heat exchanger 40 also adaptedto a gas turbine, embodying the invention but constructed somewhatdifferently. As in the heat exchanger 14, it is essentially a compositeof several liquid metal to gas heat exchangers with the liquid metalflowing in cross-flow to the gas and the hot gas from the turbineexhaust flowing through sections 41 counter to the flow of cold gas fromthe compressor through intermediate sections 42.

In this form, the 'heat exchanger bears a closer resemblance to a rotaryregenerator, the essential difference being that heat is transportedfrom the hot sections 41 to the cold sections 42 by circumferentialmotion of fluid within tubes rather than by physical rotation of theheat transfer elements which are necessary features of the conventionalrotary regenerator.

FIG. 7 illustrates diagrammatically one possible arrangement in which atube 43 is indicated as a single continuous tube in one plane or discconnected with a pump 45. The hot and cold gases flow axially throughpreferably triangularly shaped passages 46. A plurality of adjacentparallel tubes 43 would be provided as shown in FIG. 12 to occupy thedesired axial length, so that operational temperature differencesbetween adjacent circumferential segments in each plane will be smallalthough temperature diiferences in the axial direction of theregenerator will have the conventional differences associated with heattransfer principles. The temperature excursion is illustrated in FIG.11, in which the rise and fall of temperature within each pass orcircuit of fluid flow in each plane will be relatively small, and meantemperature of the several passes will have the differences indicated,the pass at the Ihot gas inlet side being to the left in the chart. Thistemperature excursion will be generally the same regardless of thearrangement of heat transport fluid ducting and headers used.

In accomplishing heat transfer in and out of each tube 43, the fluidflow rate is maintained at a circumferential velocity generallycomparable to the matrix motion in a rotary regenerator. In doing this,it is apparent that a slug of fluid will be heated by the hot gases inthe time interval required to pass a distance equal to the pitch ofradial separators 44. In passing into the cold section 42, this heatwill be transferred to the cold air stream in accordance withconventional heat transfer principles. The cycle is then repeatedcontinuously.

When there is an appreciable change in radius between the inner andouter coils of tubes 43, it will be seen that a preferred mean cycle maybe arranged for the mean diameter tube cycle. The fluid transportrotational speed will vary, with slowest angular rotation occuring inthe larger outer diameter length of tube and fastest rotation occurringin the smaller inner diameter length of tube. In this sense, the dwelltime for the present heat exchanger will diflfer from the dwell time ina rotary regenerator.

, Any other tube may of course be selected for the optimum dwell speed.The use of low conductivity thin wall tubing such as stainless steelwill reduce circumferential heat transfer to a minimum. There is noapparent great disadvantage in varying the dwell time over a limitedrange of variation from the mean dwell time. Some increases in size ofthe heat exchanger will result from deviation from an optimum dwelltime, but this is compensated for by the simplicity of the system.

FIG. 9 illustrates diagrammatically a fragment of a heat exchanger 47 inwhich the heat transfer fluid tubes 48 are indicated as being contouredto permit equal passage time through all respectively radially spacedportions of the tubing, eliminating variations in fluid transportrotational speed and thus giving more uniform heat transfer efficiencyover the radial dimension.

FIG. 8 illustrates diagrammatically a consolidated heat exchanger 49having a plurality of sections adapted for connection with severalconsecutive heat transfer services as may be needed in a gasturbineengine. Heat transfer fluid tubes have been omitted for clarity, but itwill be apparent that they will be generally similar to those indicatedin FIG. 7.

It will be seen from the foregoing descriptions that the present heatexchanger accomplishes a close union of the essential elements of thetypical fluid transport regenerator in which the hot gas-to-liquidsection heretofore has been remote from and connected by lengths ofpiping from the hot fluid to the cold air regenerator. In one sense, thepresent heat exchanger might be considered a liquid transportarrangement in which the connecting pipes have substantially zero lengthsince they extend directly from hot to cold sect-ions. It may also bethought of as consisting of series segments of a multi-pass cross flowheat exchanger in which the interconnecting header sections have beeneliminated, the headers of the structure in FIGS. 2-6 being, in effect,additional individual connecting passages disposed in the cold sections.

Thus the present device has many advantages when compared toconventional heat exchangers. It is a structurally sound element that isreadily adapted to engine structure. It eliminates the disadvantages ofthe rotary regenerator, viz: it needs no mechanical drive, hence nobearing and gears; it eliminates leakage between gas passages and avoidstroublesome seals; it has no carry-over of the gases from one passage tothe other and can thus be used with radically different gases. It is notlimited to gas-to-gas heat exchange but can be readily adapted forliquid to gas, gas to liquid, and liquid to liquid heat exchange, or acombination of these may be handled by one assembly. It also eliminatesthe bulky and/or complex ducting heretofore required by static typecounter-flow heat exchangers. Moreover, the elimination of liquidtransfer ducts as are used where the hot and cold systems are separatedavoids the problem of cold weather operation in which freezing can occurin the transfer ducts. The present concept envisions surrounding all ofthe liquid metal passages with heated air which is more than adequate oncold starting to thaw the liquid metal.

In all of the embodiments of the invention presented herein, it will beseen that the device may be compared to a rotary regenerator with theextended surface of matrix on the gas side stationary and liquid metalflowing in cross-flow to the gas taking the place of the rotating mass.

In order to determine the number of transfer units (TU) or core sectionsrequired in the present device, it is necessary to first establish thetemperature distribution in the fluid streams as they proceed throughthe heat exchanger. It is apparent that the efl'ectivenessNTUrelationships which have been established for rotary regeneratorsimplicitly include consideration of the temperature distribution.

Consider the heat exchanger divided into N modules, each consisting of ahot-gas to liquid-metal and a cold-gas to liquid-metal section. Sincethe entire flow of liquid metal passes first through the hot section andthen through the cold section, and l/N times the total gas flow passesthrough each section of the module, the capacity rate ratio for themodule will be NC /C where C is the liquid capacity rate and C is thegas capacity rate (that is, we may increase the effective capacity rateratio between the liquid metal side and the gas side by increasing thenumber of modules). In the limit, as N approaches infinity, the weightof liquid metal flow required to achieve a finite capacity rate ratiotends to zero. Also, as N approaches infinity, there is a hot air passof infinitesimal width, on each side of a cold air pass. But, this issimply a direct gas-to-gas heat exchanger of the counterflow type.Therefore, we may conclude that if N and/ or C is much greater than thecapacity rate (0 of the gas flow, the performance of the heat exchangerwill be that of a counterflow heat exchanger.

Using a capacity rate ratio of five between the liquid metal and the gasfor each module and C /C equal to 1.0 where C is the capacity rate ofcold gas and C is the capacity rate of hot gas, measured inB.t.u./sec.R., it may be seen from FIG. 14, that the NTU required for aneffectiveness of 0.80 is 4.25. It is interesting to note that across-flow heat exchanger of C /C equal to 1.0 with an NTU of 4.25 willhave an effectiveness of 0.75; and

that a cross-flow heat exchanger with an eifectiveness of 0.8 wouldrequire an NTU of approximately eight (8). Thus, it is apparent that inorder to minimize the heat exchanger size (of which NTU is a measure fora given gas flow rate) it is highly desirable to achieve a counterflowarrangement.

FIG. 13 presents the results of performance analysis in the form ofeffectiveness (AP/P) or cold gas pressure drop and (AP/P)h or hot gaspressure drop being plotted versus frontal area, A It will be noted(lowermost set of curves) that for a gas flow W of 5.6 pounds persecond, it is possible to achieve an elfectiveness of 0.8 with a frontalarea of 6.5 square feet and an air flow length of five inches. The coldside pressure drop factor at this point would be 0.0013 and the hot sidepressure drop factor would be 0.0355. The total AP/P would be 0.368. Itshould be noted that this pressure drop is core friction only, and noeffort has been made to account for the entrance or exit pressure lossesor flow acceleration. The hot side pressure drop (which is more than tentimes the cold side pressure drop) will not be adversely affected byflow acceleration, since the gas is being cooled and hence anyacceleration term would tend to reduce the calculated pressure drop. Afinal design solution of the problem would of course include calculationof these pressure drop terms.

Depending upon the type of liquid metal pump available, one may connectthe liquid metal passages in series or parallel since either scheme willresult in the same flow rate of metal passing a given cross-section ofthe heat exchanger. If a high head, low volume flow pump is morefeasible than a low head, high flow pump, it will probably be desirableto connect the passages in series.

Although the required power is very low, every effort has been made tobe conservative in this calculation. One explanation for the low powerrequired is that with N=4, which is the specific case calculated, thereis a relatively low value for C,,,,,,/ N and the liquid metal flow raterequired to achieve NC /C equal to 5 is therefore also low.

The heat exchanger weight has also been calculated theoretically. Using0.004 thick stainless steel, the core density is 98 lbm./ft. There willbe an additional 6.35 lbm./ft. of liquid metal if Nak (56 percent Na 44percent K) at l200 R. is used. The total density is thus 104.85 lbm./ftA unit with a frontal area A of 6.5 ft. and length L of 5.0 inch wouldweigh 281 lbm. This is 0.468 lbm./H1. weight penalty for theregenerator.

Although it may be possible to use fin and plate material thinner than0.004 inch, a conservative approach has been taken. The use oftriangular fin results in the least weight for a given fin thickness andunit performance.

The performance shown in FIG. 13 is for a unit with mild steel fins.Mild steel has a conductivity of 23 B.t.u./ hr. R. ft. compared to 32B.t.u./hr. F. cfor nickel.

The foregoing analysis is based on the assumption of a capacity rateratio between the hot and cold gases of nearly 1.0. This assumption isvalid since the air fuel ratio in the particular gas turbine applicationconsidered is approximately 840: 1.

Although only a few embodiments of the invention are described, it willbe apparent to one skilled in the art to which the invention pertainsthat various changes and modifications may be made therein withoutdeparting from the spirit of the invention or the scope of the appendedclaims.

I claim:

1. A heat exchanger comprising (a) a core structure having a pluralityof closely spaced parallel passages extending from one face to anopposite face of said structure,

(b) a hot fluid duct and a cold fluid duct connected with adjacent areasof said core structure and adapted to provide respectively hot and coldfluid flow through adjacent groups of said passages,

(c) means transferring heat from the areas of said core structureconnected with said hot fluid duct to the areas of said core structureconnected with said cold fluid duct and comprising a plurality of fluidconduits extendingthrough said core structure areas normal to saidpassages and in heat exchange relation therewith, and means circulatinga heat transporting fluid through said fluid conduits simultaneouslywith the flow of fluid through said hot and cold fluid ducts.

2. The heat exchanger as defined in claim 1 and in which said fluidconduits are arranged in a plurality of spaced transverse levels and areconnected to headers at opposite ends, said headers being arranged todirect fluid through a group of said conduit levels in one direction andsubsequently through a group of conduit levels in the oppositedirection.

3. The heat exchanger as defined in claim :1 and in which said fluidconduits are arranged in a plurality of spaced transverse levels and areconnected to headers at opposite ends, said headers being arranged todirect fluid in repeated passes through said conduit levels, said levelsbeing arranged in groups and the fluid passing through adjacent groupsbeing directed in opposite directions.

4. The heat exchanger as defined in claim 1 and in which said fluidconduits are arranged in a plurality of spaced transverse levels andhaving a corrugated sheet disposed intermediate and spacing said conduitlevels to provide adjacent substantially triangular passages betweensaid levels.

5. In combination with the gas turbine engine having an air inductionmeans and an exhaust gas conducting means, means for transferring theheat from the exhaust gases carried by said exhuast gas conducting meansto the air conducted by said air induction means comprising (a) a firstheat exchanger and a second heat exchanger, said heat exchangers havinga substantially common side wall,

(b) said first heat exchanger being connected with said air inductionmeans,

() said second heat exchanger being connected with said exhaust gasconducting means,

(d) conduits passing through said side wall and said heat exchangers andmeans circulating a heat transporting fluid through said conduits inheat exchange relation with and in cross flow relation to both of thefluids conducted through said heat exchangers.

6. A heat exchanger comprising (a) a hot fluid conducting duct and acold fluid conducting duct;

(b) said ducts having a substantially common sidewall;

(c) means transferring heat from said hot fluid conducting duct to saidcold fluid conducting duct and comprising fluid conduits passing throughsaid side wall and said ducts and means circulating a heat transportingfluid through said conduits simultaneously and in heat exchange relationwith and in cross flow relation to the fluids conducted through saidducts; and

(d) said heat exchanger having a plurality of said hot ducts and aplurality of cold ducts each arranged intermediate two of said hotducts, said ducts being arranged parallel to each other to form acircumferential drum, and said conduits arranged in coils to extendthrough said ducts.

References Cited by the Examiner UNITED STATES PATENTS 2,413,225 12/1946 Griflith 39.66 2,469,028 5/ 1949 Balaiefl '165140 X 2,650,0738/1953 Holm -140 2,674,849 4/ 1954 Bowden 60--39.66 2,731,239 1/ 1956Andersen 6039.5l 2,995,344 8/ 1961 Hryniszak 257-245 3,138,925 6/1964Machery 6039.18

FOREIGN PATENTS 668,493 12/ 1938 Germany. 1,088,027 9/ 1960 Germany.

734,938 8/ 1955 Great Britain.

ROBERT A. OLEARY, Primary Examiner.

HERBERT L. MARTIN, CHARLES SUKALO, FRED- ERICK L. MATTESON, Examiners.

T. W. STREULE, Assistant Examiner.

1. A HEAT EXCHANGER COMPRISING (A) A CORE STRUCTURE HAVING A PLURALITYOF CLOSELY SPACED PARALLEL PASSAGES EXTENDING FROM ONE FACE TO ANOPPOSITE FACE OF SAID STRUCTURE, (B) A HOT FLUID DUCT AND COLD FLUIDDUCT CONNECTED WITH ADJACENT AREAS OF SAID CORE STRUCTURE AND ADAPTED TOPROVIDE RESPECTIVELY HOT AND COLD FLUID FLOW THROUGH ADJACENT GROUPS OFSAID PASSAGES, (C) MEANS TRANSFERRING HEAT FROM THE AREAS OF SAID CORESTRUCTURE CONNECTED WITH SAID HOT FLUID DUCT TO THE AREAS OF SAID CORESTRUCTURE CONNECTED WITH SAID COLD FLUID DUCT AND COMPRISING A PLURALITYOF FLUID CONDUITS EXTENDING THROUGH SAID CORE STRUCTURE AREAS NORMAL TOSAID PASSAGES AND IN HEAT EXCHANGE RELATION THEREWITH, AND MEANSCIRCULATING A HEAT TRANSPORTING FLUID THROUGH SAID FLUID CONDUITSSIMULTANEOUSLY WITH THE FLOW OF FLUID THROUGH SAID HOT AND COLD FLUIDDUCTS.